1. Field of the Invention
The present invention relates to a variable gain amplifier, and, more specifically, relates to a variable gain amplifier which can be used as the AGC circuit of the signal receiving unit in the CDMA (code division multiple access) system digital cellular telephone, for example.
2. Description of the Related Art
Heretofore, a variable gain amplifier has been widely used as an AGC circuit (automatic gain control) to control gains in order to obtain a fixed output signal level for a fluctuating modulated input signal level. The reliability of demodulation is increased by applying a fixed signal level to the demodulator at the later stage of processing, and high signal-to-noise ratio (S/N ratio), i.e. small signal distortion, also increases the reliability of demodulation.
More particularly, when an input signal level to the variable gain amplifier is small, an AGC circuit having low noise characteristics is required to obtain the high signal-to-noise ratio described above. When the input signal level is large, a variable gain amplifier with a high saturation input level is required so that the output signal is not distorted.
In general, in order to expand the dynamic range of an amplifier from the noise level generated in the amplifier to the saturation input level at which the output signal is distorted, a low noise transistor is used to obtain the low noise characteristic and an amplifier having a negative feedback structure is used to increase the saturation input level.
However, in the case where a negative feedback structure is applied to the amplifier in order to expand the dynamic range, the saturation input level is improved, but the noise characteristic of the amplifier becomes worse. As a result, even if a low noise transistor is used, the low noise characteristic is not compatible with the, increase in saturation input level.